The ATLAS and CMS experiments observed a particle at the LHC with a mass ≈126 GeV, which is compatible with the Higgs boson of the Standard Model. A crucial question is, if for such a Higgs mass ...value, one could extrapolate the model up to high scales while keeping the minimum of the scalar potential that breaks the electroweak symmetry stable. Vacuum stability requires indeed the Higgs boson mass to be MH≳129±1 GeV, but the precise value depends critically on the input top quark pole mass which is usually taken to be the one measured at the Tevatron, mtexp=173.2±0.9 GeV. However, for an unambiguous and theoretically well-defined determination of the top quark mass one should rather use the total cross section for top quark pair production at hadron colliders. Confronting the latest predictions of the inclusive pp¯→tt¯+X cross section up to next-to-next-to-leading order in QCD to the experimental measurement at the Tevatron, we determine the running mass in the MS¯-scheme to be mtMS¯(mt)=163.3±2.7 GeV which gives a top quark pole mass of mtpole=173.3±2.8 GeV. This leads to the vacuum stability constraint MH⩾129.4±5.6 GeV to which a ≈126 GeV Higgs boson complies as the uncertainty is large. A very precise assessment of the stability of the electroweak vacuum can only be made at a future high-energy electron–positron collider, where the top quark pole mass could be determined with a few hundred MeV accuracy.
NLO PDFs from the ABMP16 fit Alekhin, S.; Blümlein, J.; Moch, S.
The European physical journal. C, Particles and fields,
06/2018, Letnik:
78, Številka:
6
Journal Article
Recenzirano
Odprti dostop
We perform a global fit of parton distribution functions (PDFs) together with the strong coupling constant
α
s
and the quark masses
m
c
,
m
b
and
m
t
at next-to-leading order (NLO) in QCD. The ...analysis applies the
MS
¯
renormalization scheme for
α
s
and all quark masses. It is performed in the fixed-flavor number scheme for
n
f
=
3
,
4
,
5
and uses the same data as the previous ABMP16 fit at next-to-next-to-leading order (NNLO). The new NLO PDFs complement the set of ABMP16 PDFs and are to be used consistently with NLO QCD predictions for hard scattering processes. At NLO we obtain the value
α
s
(
n
f
=
5
)
(
M
Z
)
=
0.1191
±
0.0011
compared to
α
s
(
n
f
=
5
)
(
M
Z
)
=
0.1147
±
0.0008
at NNLO.
We present a detailed comparison of the fixed-order predictions computed by four publicly available computer codes for Drell–Yan processes at the LHC and Tevatron colliders. We point out that while ...there is agreement among the predictions at the next-to-leading order accuracy, the predictions at the next-to-next-to-leading order (NNLO) differ, whose extent depends on the observable. The sizes of the differences in general are at least similar, sometimes larger than the sizes of the NNLO corrections themselves. We demonstrate that the neglected power corrections by the codes that use global slicing methods for the regularization of double real emissions can be the source of the differences. Depending on the fiducial cuts, those power corrections become linear, hence enhanced as compared to quadratic ones that are considered standard.
We consider determinations of the strange sea in the nucleon based on QCD analyses of data collected at the LHC with focus on the recent high-statistics ATLAS measurement of the W±- and Z-boson ...production. We study the effect of different functional forms for parameterization of the parton distribution functions and the combination of various data sets in the analysis. We compare to earlier strange sea determinations and discuss ways to improve them in the future.
We study the production of heavy quarks in deep-inelastic scattering within perturbative QCD. As a new result, we employ for the first time the running mass definition in the MS¯ scheme for ...deep-inelastic charm and bottom production. We observe an improved stability of the perturbative expansion and a reduced theoretical uncertainty due to variations of the renormalization and factorization scales. As our best estimate we extract from a global fit to fixed-target and HERA collider data for the charm-quark an MS¯ mass of mc(mc)=1.01±0.09(exp)±0.03(th)GeV.
We review the present status of the determination of parton distribution functions (PDFs) in the light of the precision requirements for the LHC in Run 2 and other future hadron colliders. We provide ...brief reviews of all currently available PDF sets and use them to compute cross sections for a number of benchmark processes, including Higgs boson production in gluon–gluon fusion at the LHC. We show that the differences in the predictions obtained with the various PDFs are due to particular theory assumptions made in the fits of those PDFs. We discuss PDF uncertainties in the kinematic region covered by the LHC and on averaging procedures for PDFs, such as advocated by the PDF4LHC15 sets, and provide recommendations for the usage of PDF sets for theory predictions at the LHC.
PDF
